Senate Democrats set up for a long night of speechmaking to protest a GOP blockade against a bill to increase campaign finance disclosure of super PACs.

After Senators rejected, 51-44, a Democratic attempt to move forward on the legislation known as the DISCLOSE Act, Democrats sought to publicize the filibuster through a series of evening floor speeches, which they called a “midnight vigil.” Sixty votes were needed to overcome GOP objections.

Majority Leader Harry Reid (D-Nev.) switched his vote to “no” to allow him to call a repeat vote Tuesday.

Reid blasted Republicans for their unanimous opposition in a speech Monday afternoon on the Senate floor, attempting to tie the bill to the issue of whether presumptive GOP presidential nominee Mitt Romney will release years of tax returns. Romney has so far declined to do so.

“My Republican colleagues — with rare exception — have lined up against this common-sense legislation. Their newfound opposition to transparency makes one wonder who they’re trying to protect,” Reid said.

“Perhaps Republicans want to shield the handful of billionaires willing to contribute nine figures to sway a close presidential election?” Reid added. “These donors have something in common with their nominee. Like Mitt Romney, they believe they play by their own set of rules.”

Bill sponsor Sen. Sheldon Whitehouse (D-R.I.) explained the late-night session as necessary to highlight the need for the bill. “Putting an end to secret election spending by special interests is an essential step in protecting middle class priorities. For that reason, we are committed to continuing the debate on the DISCLOSE Act late into the night and asking for a second vote tomorrow if need be,” Whitehouse said in a statement before the vote. “We can’t let the special interests off the hook after just one round.”

The legislation would require covered entities to report donations of $10,000 or more to their political activities, in a bid to provide new sunlight to the funding of third-party expenditure groups known as super PACs.

Officially titled the Democracy Is Strengthened by Casting Light On Spending in Elections Act, the bill, Democrats said, is needed to make campaign spending more transparent after the 2010 Citizens United decision by the Supreme Court that allowed corporate spending on political campaigns.

Senate Minority Leader Mitch McConnell (R-Ky.) has been the most vocal critic of the legislation, declaring it an affront to free speech. McConnell said requiring organizations involved in campaigning to disclose donor lists could lead to improper intimidation.

“The creation of a modern-day Nixonian enemies list is currently in full swing, and frankly, the American people should not stand for it,” McConnell said on the Senate floor. “As I’ve said before, no individual or group in this country should have to face harassment or intimidation, or incur crippling expenses defending themselves against their own government, simply because that government doesn’t like the message they’re advocating. But that’s what we’re seeing.”

Outside business groups and the National Rifle Association have opposed the DISCLOSE measure, saying it could interfere with the free speech of their members while providing favorable treatment to labor unions, a charge that Democrats have repeatedly denied.

That opposition from GOP-leaning groups appears to ensure the futility of the attempt at repeated votes on the measure in this Congress.